CD34-positive early human thymocytes: T cell receptor and cytokine receptor gene expression

γδ T cells: origin and fate, subsets, diseases and immunotherapy

The intricacy of diseases, shaped by intrinsic processes like immune system exhaustion and hyperactivation, highlights the potential of immune renormalization as a promising strategy in disease treatment. In recent years, our primary focus has centered on γδ T cell-based immunotherapy, particularly pioneering the use of allogeneic Vδ2+ γδ T cells for treating late-stage solid tumors and tuberculosis patients. However, we recognize untapped potential and optimization opportunities to fully harness γδ T cell effector functions in immunotherapy. This review aims to thoroughly examine γδ T cell immunology and its role in diseases. Initially, we elucidate functional differences between γδ T cells and their αβ T cell counterparts. We also provide an overview of major milestones in γδ T cell research since their discovery in 1984. Furthermore, we delve into the intricate biological processes governing their origin, development, fate decisions, and T cell receptor (TCR) rearrangement within the thymus. By examining the mechanisms underlying the anti-tumor functions of distinct γδ T cell subtypes based on γδTCR structure or cytokine release, we emphasize the importance of accurate subtyping in understanding γδ T cell function. We also explore the microenvironment-dependent functions of γδ T cell subsets, particularly in infectious diseases, autoimmune conditions, hematological malignancies, and solid tumors. Finally, we propose future strategies for utilizing allogeneic γδ T cells in tumor immunotherapy. Through this comprehensive review, we aim to provide readers with a holistic understanding of the molecular fundamentals and translational research frontiers of γδ T cells, ultimately contributing to further advancements in harnessing the therapeutic potential of γδ T cells.

Development of γδ T cells in the thymus - A human perspective

γδ T cells are increasingly emerging as crucial immune regulators that can take on innate and adaptive roles in the defence against pathogens. Although they arise within the thymus from the same hematopoietic precursors as conventional αβ T cells, the development of γδ T cells is less well understood. In this review, we focus on summarising the current state of knowledge about the cellular and molecular processes involved in the generation of γδ T cells in human.

From the cradle to the grave: activities of GATA-3 throughout T-cell development and differentiation

GATA family transcription factors play multiple vital roles in hematopoiesis in many cell lineages, and in particular, T cells require GATA-3 for execution of several developmental steps. Transcriptional activation of the Gata3 gene is observed throughout T-cell development and differentiation in a stage-specific fashion. GATA-3 has been described as a master regulator of T-helper 2 (Th2) cell differentiation in mature CD4(+) T cells. During T-cell development in the thymus, its roles in the CD4 versus CD8 lineage choice and at the β-selection checkpoint are the best characterized. In contrast, its importance prior to β-selection has been obscured both by the developmental heterogeneity of double negative (DN) 1 thymocytes and the paucity of early T-lineage progenitors (ETPs), a subpopulation of DN1 cells that contains the most immature thymic progenitors that retain potent T-lineage developmental potential. By examining multiple lines of in vivo evidence procured through the analysis of Gata3 mutant mice, we have recently demonstrated that GATA-3 is additionally required at the earliest stage of thymopoiesis for the development of the ETP population. Here, we review the characterized functions of GATA-3 at each stage of T-cell development and discuss hypothetical molecular pathways that mediate these functions.

Adoptive precursor cell therapy to enhance immune reconstitution after hematopoietic stem cell transplantation in mouse and man

Hematopoietic stem cell transplantation is a curative therapy for hematological malignancies. T cell deficiency following transplantation is a major cause of morbidity and mortality. In this review, we discuss adoptive transfer of committed precursor cells to enhance T cell reconstitution and improve overall prognosis after transplantation.

Different thresholds of Notch signaling bias human precursor cells toward B-, NK-, monocytic/dendritic-, or T-cell lineage in thymus microenvironment

Notch receptors are involved in lineage decisions in multiple developmental scenarios, including hematopoiesis. Here, we treated hybrid human-mouse fetal thymus organ culture with the gamma-secretase inhibitor 7 (N-[N-(3,5-difluorophenyl)-l-alanyl]-S-phenyl-glycine t-butyl ester) (DAPT) to establish the role of Notch signaling in human hematopoietic lineage decisions. The effect of inhibition of Notch signaling was studied starting from cord blood CD34(+) or thymic CD34(+)CD1(-), CD34(+)CD1(+), or CD4ISP progenitors. Treatment of cord blood CD34(+) cells with low DAPT concentrations results in aberrant CD4ISP and CD4/CD8 double-positive (DP) thymocytes, which are negative for intracellular T-cell receptor beta (TCRbeta). On culture with intermediate and high DAPT concentrations, thymic CD34(+)CD1(-) cells still generate aberrant intracellular TCRbeta(-) DP cells that have undergone DJ but not VDJ recombination. Inhibition of Notch signaling shifts differentiation into non-T cells in a thymic microenvironment, depending on the starting progenitor cells: thymic CD34(+)CD1(+) cells do not generate non-T cells, thymic CD34(+)CD1(-) cells generate NK cells and monocytic/dendritic cells, and cord blood CD34(+)Lin(-) cells generate B, NK, and monocytic/dendritic cells in the presence of DAPT. Our data indicate that Notch signaling is crucial to direct human progenitor cells into the T-cell lineage, whereas it has a negative impact on B, NK, and monocytic/dendritic cell generation in a dose-dependent fashion.

Repression of transcription at the human T-cell receptor Vβ2.2 segment is mediated by a MAX/MAD/mSin3 complex acting as a scaffold for HDAC activity

The identification of protein components in complex networks of co-regulators responsible for the modulation of proliferation versus differentiation modes of cell growth is a major problem. We use a combination of surface enhanced laser desorption/ionization mass spectrometry, surface plasmon resonance coupled to electrospray mass spectrometry, and immunoelectromobility shift assays to identify members of the MAX/MAD family binding to a specific DNA silencer fragment involved in the regulation of transcription for the human T-cell receptor Vbeta2.2 segment. We also identify the cofactors mSin3 and N-CoR known to interact with histone deacetylases. Inhibition of deacetylase activity in Jurkat cells prevented transcription inhibitor complex formation at the Vbeta2.2 segment, suggesting that this is either directly or indirectly dependent on the presence of HDACs.

Regulation of surface expression of the human pre-T cell receptor complex

Considerable progress has recently been made in defining the role that pre-antigen receptor complexes, namely the pre-T and pre-B cell receptors, play in lymphocyte development. It is now established that these receptors direct, in a similar way, the survival, expansion, clonality and further differentiation of pre-T and pre-B lymphocytes, respectively. However, less is known about the mechanisms which ensure that only minute amounts of pre-TCR and pre-BCR reach the plasma membrane of developing lymphocytes. In this review, we discuss the implications of recent experimental approaches which address the developmental regulation of human pre-TCR expression and the molecular mechanisms that control surface pre-TCR expression levels.

CDw108 expression during T-cell development

We recently reported a gene encoding the human CDw108, a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein that is preferentially expressed on activated T lymphocytes and erythrocytes. The present study investigated the expression of CDw108 on various tissues and cells, particularly on T cells during development. The murine CDw108 cDNA was cloned initially, and it was highly homologous to the human CDw108 (88.0% or 89.3% similarity at the nucleotide or amino acid level, respectively) or identical to the murine semaphorin K1/Sema7A. The CDw108 mRNA was demonstrated in a few tissues including thymus and brain with the highest expression coming on day 7 in whole embryo followed by relatively consistent expression during development. Cell-surface expression of the CDw108 during T-cell development was further examined by flow cytometry in the human umbilical cord blood and thymus. It was preferentially expressed on a CD34+ stem cell population of umbilical cord blood, and CD3dull CD34+/- CD117 (c-kit)+ CD4bright CDbright cells in the thymus that are involved in the stage of positive selection. These results suggest the contribution of CDw108 in T-cell development, especially in the stage of positive selection in the thymus.

T-cell development in human thymoma

Human thymoma is derived from thymic epithelial cells and often associated with a large number of cortical thymocytes. Since thymic epithelial cells play key roles in T-cell development in the normal thymus, we hypothesized that the neoplastic epithelial cells of thymoma may support T-cell differentiation. We attempted to reconstitute the T-cell development in vitro by using neoplastic epithelial cells isolated from thymoma. CD34, a stem cell marker, was expressed on a proportion of CD4-CD8- cells in thymoma. These CD34+CD4-CD8- cells also expressed both IL-7R alpha-chain and common gamma-chain. Purified CD4-CD8- cells from thymomas were cultured with the neoplastic epithelial cells, and their differentiation into CD4+CD8+ cells via CD4 single positive intermediates was observed within 9 days' co-culture in the presence of recombinant IL-7. The CD34+CD4-CD8- cells purified from a normal thymus also differentiated to CD4+CD8+ cells in an allogeneic co-culture with the neoplastic epithelial cells of thymoma. In addition, a pleural dissemination from thymoma contained a large amount of cortical thymocytes. These results suggest that the neoplastic epithelial cells retain the function of thymic epithelium and can support T-cell development in thymomas.

The majority of lymphocytes in the bone marrow, thymus and extrathymic T cells in the liver are generated in situ from their own preexisting precursors

Parabiotic pairs of B6.Ly5.1 and B6.Ly5.2 mice were used to investigate how lymphocytes in various organs and various lymphocyte subsets mixed with partner cells. The origin of partner cells was determined by using anti-Ly5.1 mAb in conjunction with immunofluorescence tests. Parabiosis was also produced after the irradiation of B6.Ly5.2 mice at various doses to prepare an immunosuppressive partner. Irrespective of irradiation, lymphocytes and other hematopoietic cells in the bone marrow and lymphocytes in the thymus showed a low mixture of partner cells in comparison with those of all other organs tested. On the other hand, lymphocytes in the blood, spleen, and lymph nodes became a half-and-half mixture of their own cells and partner cells by 14 days after parabiosis. Among lymphocyte subsets, intermediate CD3 cells (i.e., CD3int cells) and NKT cells (i.e., NK1.1+ subset of CD3int cells) in the liver also showed a low mixture of partner cells. The present results raise the possibility that lymphocytes in the bone marrow and thymus, and extrathymic T cells in the liver might be in situ generated from their own preexisting precursor cells. Another observation was that, after irradiation, partner cells showed accelerated mixture even if they showed a low mixture under non-irradiated conditions. However, only lymphocyte subsets with the same phenotype as those of preexisting cells entered the corresponding sites.

The c-kit receptor and its ligand stem cell factor in childhood malignant lymphoid precursors

The c-kit receptor (CD117) and its ligand stem cell factor (SCF) play an important role in the development, differentiation, and survival of normal and malignant hematopoietic cells. The aim of this work is to review the cellular distribution of this receptor and the effect of SCF on the hematopoietic system, particularly among lymphoid lineage, either in normal or malignant cell progenitors. We examined reports and results in the field and articles or abstracts published in journals covered by MEDLINE. Additionally, we evaluated CD117 expression on fresh blast cells of 376 newly diagnosed cases of childhood acute lymphoblastic leukemia (ALL) that were referred to centers affiliated with the Italian Association for Pediatric Hematology and Oncology (AIEOP). In view of our data, approximately 11% of ALL are CD117 positive. In particular, this receptor can be expressed in 10% and 11.5% of T-lineage and B-lineage ALL, respectively. Its expression is associated with an intermediate/mature phenotype in T-lineage ALL, whereas in B-lineage ALL, the majority of the positive cases are classified as early B ALL. The effect of SCF on malignant hematopoiesis and its potential clinical uses are reviewed.

Unusual immunoglobulin and T-cell receptor gene rearrangement patterns in acute lymphoblastic leukemias

Immunoglobulin (Ig) and T-cell receptor (TCR) genes are rearranged in virtually all acute lymphoblastic leukemia (ALL) cases. However, the recombination patterns display several unusual features as compared to normal lymphoid counterparts. Cross-lineage gene rearrangements occur in more than 90% of precursor-B-ALL and in approximately 20% of T-ALL, whereas they are rare in normal lymphocytes. Approximately 25-30% of the Ig and TCR gene rearrangements at diagnosis are oligoclonal, and can undergo continuing or secondary recombination events during the disease course. Based on our extensive molecular studies we hypothesize that the unusual Ig and TCR gene rearrangements in ALL occur as an early postoncogenic event resulting from the continuing V(D)J recombinase activity on accessible gene loci. This hypothesis is on the one hand supported by the virtual absence of cross-lineage gene rearrangements in normal lymphocytes and mature lymphoid malignancies and on the other hand by the presence of oligoclonality and secondary Ig and TCR gene rearrangements in ALL.

TCR Gene Rearrangements and Expression of the Pre-T Cell Receptor Complex During Human T-Cell Differentiation

Recent studies have identified several populations of progenitor cells in the human thymus. The hematopoietic precursor activity of these populations has been determined. The most primitive human thymocytes express high levels of CD34 and lack CD1a. These cells acquire CD1a and differentiate into CD4+CD8+ through CD3−CD4+CD8− and CD3−CD4+CD8+β− intermediate populations. The status of gene rearrangements in the various TCR loci, in particular of TCRδ and TCRγ, has not been analyzed in detail. In the present study we have determined the status of TCR gene rearrangements of early human postnatal thymocyte subpopulations by Southern blot analysis. Our results indicate that TCRδ rearrangements initiate in CD34+CD1a− cells preceding those in the TCRγ and TCRβ loci that commence in CD34+CD1a+ cells. Furthermore, we have examined at which cellular stage TCRβ selection occurs in humans. We analyzed expression of cytoplasmic TCRβ and cell-surface CD3 on thymocytes that lack a mature TCRβ. In addition, we overexpressed a constitutive-active mutant of p56lckF505 by retrovirus-mediated gene transfer in sequential stages of T-cell development and analyzed the effect in a fetal thymic organ culture system. Evidence is presented that TCRβ selection in humans is initiated at the transition of the CD3−CD4+CD8− into the CD4+CD8+β− stage.

TCR Gene Rearrangements and Expression of the Pre-T Cell Receptor Complex During Human T-Cell Differentiation

Recent studies have identified several populations of progenitor cells in the human thymus. The hematopoietic precursor activity of these populations has been determined. The most primitive human thymocytes express high levels of CD34 and lack CD1a. These cells acquire CD1a and differentiate into CD4+CD8+ through CD3−CD4+CD8− and CD3−CD4+CD8+β− intermediate populations. The status of gene rearrangements in the various TCR loci, in particular of TCRδ and TCRγ, has not been analyzed in detail. In the present study we have determined the status of TCR gene rearrangements of early human postnatal thymocyte subpopulations by Southern blot analysis. Our results indicate that TCRδ rearrangements initiate in CD34+CD1a− cells preceding those in the TCRγ and TCRβ loci that commence in CD34+CD1a+ cells. Furthermore, we have examined at which cellular stage TCRβ selection occurs in humans. We analyzed expression of cytoplasmic TCRβ and cell-surface CD3 on thymocytes that lack a mature TCRβ. In addition, we overexpressed a constitutive-active mutant of p56lckF505 by retrovirus-mediated gene transfer in sequential stages of T-cell development and analyzed the effect in a fetal thymic organ culture system. Evidence is presented that TCRβ selection in humans is initiated at the transition of the CD3−CD4+CD8− into the CD4+CD8+β− stage.

In Vitro Intrathymic Differentiation Kinetics of Human Fetal Liver CD34+CD38− Progenitors Reveals a Phenotypically Defined Dendritic/T-NK Precursor Split

Human CD34+CD38− hematopoietic precursor cells from fetal liver are able to develop into T, NK, and dendritic cells in a hybrid human/mouse fetal thymic organ culture (FTOC). In this report, we pay particular attention to the early events in differentiation of these precursor cells. We show that the CD34+CD38− precursor cells, which are CD4−CD7−cyCD3−HLA-DR−/++ (cy, cytoplasmatic), differentiate into a CD4+ population that remained CD7−cyCD3−HLA-DR++ and a CD4− population that expressed CD7 and cyCD3. The CD4+CD7−cyCD3− cells differentiate into phenotypically and functionally mature dendritic cells, but do not differentiate into T or NK cells. The CD4−CD7+cyCD3+ population later differentiates into a CD4+CD7+cyCD3+HLA-DR− population, which has no potential to differentiate into dendritic cells but is able to differentiate into NK cells and γδ and αβ T lymphocytes. These findings support the notion that the T/NK split occurs downstream of the NK/dendritic split.

Thymosin-alpha1 stimulates maturation of CD34+ stem cells into CD3+4+ cells in an in vitro thymic epithelia organ coculture model

The effect of thymosin-alpha1 on thymopoiesis is largely unknown. Thymosin is found in the cortical and medullary thymic epithelia, as well as in nurse cells; thus, it is hypothesized that thymosin may affect both early and late stage of thymocyte maturation. In this study, the effect of thymosin-alpha1 on thymopoiesis was determined by coculturing in vitro CD34+ stem cells (SC) with allogeneic cultured thymic epithelia fragments (CTEF) for 1-4 weeks and analyzing T-cell maturation by flow cytometry. Thymosin-alpha1 significantly enhanced the cell number (e.g., proliferation) of mononuclear cells obtained at 2 and 4 weeks of the SC-CTEF cocultures (P < 0.01 and < 0.05, respectively). In particular, thymosin-alpha1 stimulated expression of CD3+ cells at 3 and 4 weeks (P < 0.05). The predominant subpopulation increased by thymosin stimulation was single positive mature CD4+ cells, which was confirmed to occur within the SC-CTEF thymic organ tissue by laser confocal immunofluorescence microscopy. Thymosin stimulation tended to enhance IL-7 synthesis, critical cytokine in the maturation of thymocytes. In summary, this is the first study to demonstrate that thymosin-alpha1 enhanced thymopoiesis of CD34+ stem cells in humans using an in vitro model of differentiation using stem cells and cultured thymic epithelial fragments cocultures. Furthermore, the thymosin significantly increased expression of CD3+4+ T cells.

Low level of mixing of partner cells seen in extrathymic T cells in the liver and intestine of parabiotic mice: its biological implication

c-kit+ stem cells have recently been found in the liver and intestine of adult mice. We examined whether such stem cells give rise to extrathymic T cells in these organs in situ. To this end, we used parabiotic B6.Ly5.1 and B6.Ly5.2 mice, i.e. mice sharing the circulation. The origin of lymphocytes was identified by anti-Ly5.1 and anti-Ly5.2 monoclonal antibodies in conjunction with immunofluorescence assays. Lymphocytes in the blood, spleen, lymph nodes and liver had become a half-and-half mixture of Ly5.1+ and Ly5.2+ cells in both individuals by day 14. However, this level of mixing decreased in extrathymic T cells in the liver (i.e. NK T cells) and intestine by day 14 and thereafter. The same was observed in T cells of the thymus. The data from immunohistochemical staining supported the results of immunofluorescence assays for suspension cells. The present results raise the possibility that extrathymic T cells in the liver and intestine may arise from their own pre-existing precursor cells, possibly from their own stem cells. Another important finding was that the composition pattern of lymphocyte subsets in one individual was quite similar to that in its partner at various sites. This result was interpreted to mean that only selected partner cells migrate to specific sites in the other partner individual.

Interleukin-7 Influences the Development of Thymic Dendritic Cells

Interleukin-7 (IL-7) has been shown to be a critical factor in B and T lymphopoiesis, and to influence the differentiation of myeloid cell lineages. In the present study we extend these results demonstrating that IL-7 also plays an important role in the development of thymic dendritic cells (DC). The addition of IL-7 to rat fetal thymus organ cultures (FTOC) resulted in a drastic increase in the number of CD3−CD4−CD8− cells, which mostly expressed typical DC markers, including major histocompatibility complex class II, OX-62, CD11b, CD68, and CD54. These cells exhibited morphological and ultrastructural features of DC, and were potent stimulators of the allogeneic mixed leukocyte reaction. Although increased numbers of DC were continuously generated throughout the culture period in the presence of IL-7, they were not actively dividing, indicating that DC in IL-7–treated cultures did not arise by expansion of pre-existing cells. Reduced DC numbers obtained after the addition of neutralizing anti–IL-7 antibodies to mouse FTOC confirmed the relevance of endogenously produced IL-7 on thymic DC development. Furthermore, the addition of IL-7 to FTOC derived from severe combined immunodeficient mice also generated large numbers of DC in the absence of thymocyte maturation.

Neoplastic thymic epithelial cells of human thymoma support T cell development from CD4-CD8- cells to CD4+CD8+ cells in vitro

Human thymoma is a thymic epithelial cell tumour which often contains a large number of immature T cells and is frequently associated with autoimmune diseases. Since thymic epithelial cells play key roles in the development and selection of T cells in the normal thymus, we hypothesized that the neoplastic thymic epithelial cells of thymoma may support T cell differentiation in the tumour. We characterized CD4-CD8- cells in thymoma and applied an in vitro reconstitution culture system using the CD4-CD8- cells and the neoplastic epithelial cells isolated from thymoma. CD34, a stem cell marker, was expressed on 29.9 +/- 12.2% of CD4-CD8- cells in thymoma. TCRgammadelta was expressed on 27.4 +/- 15.1% of CD4-CD8- cells and CD19, a B cell marker, was expressed on 14.1 +/- 23.1% of CD4-CD8- cells. CD4-CD8- cells expressed both IL-7R alpha-chain and common gamma-chain. Purified CD4-CD8- cells from thymomas were cultured with the neoplastic epithelial cells, and their differentiation into CD4+CD8+ cells via CD4 single-positive intermediates was observed within 9 days' co-culture in the presence of recombinant IL-7. Furthermore, we examined the reconstitution culture using CD34+CD4-CD8- cells purified from normal infant thymus. The CD34+CD4-CD8- cells in normal thymus also differentiated to CD4+CD8+ cells in the allogeneic co-culture with the neoplastic epithelial cells of thymoma. These results indicate that the tumour cells of thymoma retain the function of thymic epithelial cells and can induce differentiation of T cells in thymoma.

The role of interleukin-7 in early T-cell development

Interleukin-7 (IL-7) is a non redundant cytokine in thymic T-cell development. It binds to a dimeric receptor consisting of a specific IL-7Ralpha and a gamma-common subunit that it shares with the receptors for IL-2, 4, 9, 13 and 15. IL-7 is critical for early T-cell development but it also acts on immature B-cells and mature T-cells, and leads to secondary cytokine release. Its mechanisms of action in early T-cell development may be multiple. There is direct evidence to support a mechanistic involvement in TCR-gamma rearrangement that drives further TCR-gammadelta thymocyte commitment and maturation. There is indirect evidence for a role of IL-7 in TCR-beta rearrangement. It may however also act as a survival factor for TCR-beta rearranging thymocytes while the critical commitment selections are effected by other factors. The effects of IL-7 in fetal thymus organ culture are dose dependent, with a biphasic response: low doses of IL-7 are necessary for normal TCR-alphabeta thymocyte development but high doses block TCR-alphabeta maturation in favor of TCR-gammadelta development. A good understanding of the dose response of IL-7 in thymocyte development, mature T-cell stimulation, and of the release of secondary cytokines will be important for planning successful clinical trials with IL-7.

Phenotypic and immunohistological analyses of the human adult thymus: evidence for an active thymus during adult life

We analyzed cellular content of thymic samples from 26 human healthy donors, ranging from 1 week postnatal to 49 years old. Our results showed that there was an overall decrease in cellular density, beginning early during life, but with two peaks of cellular density, at 9 months and 10 years of age. Histological and immunohistological analyses showed that variations in cellular density were correlated with the morphological changes observed during thymic involution, namely the enlargement of interlobular trabeculae and the development of adipocytic tissue. However, the adult thymus still contained thymocytes, up to 49 years. Phenotypic analysis showed no significant variations according to the age of donors in the distribution of the main thymocyte subsets, both precursors and more mature cells. These results suggest that the human thymus remains active during adult life.

C-kit receptors in childhood malignant lymphoblastic cells

The product of the protooncogene c-kit is the receptor for the hematopoietic cytokine stem cell factor (SCF). C-kit is expressed on leukemic cells of the erythroid, myeloid, and mast-cell lineage and SCF has a proliferative effect on some of these cells. The role of SCF and c-kit in lymphoid malignancies is much less clear. Here we review the role of c-kit in normal lymphopoiesis and summarize its role in lymphoid malignancies. C-kit is expressed in normal lymphopoiesis and its ligand SCF synergizes with IL-7 to enhance the proliferation of B- and T-cell progenitors. In malignant lymphopoiesis, c-kit can also be expressed in B and T-lymphoblastic cells from children with non Hodgkin's lymphoma (NHL) or acute lymphoblastic leukemia (ALL) when analyzed by the highly sensitive reverse transcriptase polymerase chain reaction (RT-PCR). While c-kit receptors were detected by flow cytometric (FCM) analysis on about 40% of fresh T-lymphoblastic biopsy tumor cell preparations or T-lymphoblastic cell lines, no receptors were detected on B-lymphoblastic fresh cells or cell lines from children with B-ALL or Burkitt's lymphoma (BL). Almost all of the lymphoblastic cells expressing c-kit protein responded to recombinant human (rh)SCF with a downregulation of c-kit receptors. A proliferative response was detected only in a minority of these cells. B-ALL or BL cell lines showed no response to rhSCF. Upregulation of c-kit in T-lymphoblastic cells could be demonstrated by the addition of IL-1 beta, TNF-alpha, TGF-beta or A23187, and downregulation by rhSCF or phorbol myristate acetate (PMA). Despite upregulation of c-kit mRNA, protein remained undetectable on B-ALL or BL cells in the presence of A23187. The metabolic state of the cells seemed to influence c-kit expression, since c-kit was upregulated in T-lymphoblastic cells by the addition of new medium. C-kit appears to play a role in the growth of some malignant T-lymphoblastic but not B-lymphoblastic cells.

Regulation of pre-T cell receptor (pT alpha-TCR beta) gene expression during human thymic development

In murine T cell development, early thymocytes that productively rearrange the T cell receptor (TCR) beta locus are selected to continue maturation, before TCR alpha expression, by means of a pre-TCR alpha- (pT alpha-) TCR beta heterodimer (pre-TCR). The aim of this study was to identify equivalent stages in human thymocyte development. We show here that variable-diversity-joining region TCR beta rearrangement and the expression of full-length TCR beta transcripts have been initiated in some immature thymocytes at the TCR alpha/beta- CD4+CD8- stage, and become common in a downstream subset of TCR alpha/beta- CD4+CD8+ thymocytes that is highly enriched in large cycling cells. TCR beta chain expression was hardly detected in TCR alpha/beta- CD4+CD8- thymocytes, whereas cytoplasmic TCR beta chain was found in virtually all TCR alpha/beta- CD4+CD8+ blasts. In addition, a TCR beta complex distinct from the mature TCR alpha/beta heterodimer was immunoprecipitated only from the latter subset. cDNA derived from TCR alpha/beta- CD4+CD8+ blasts allowed us to identify and clone the gene encoding the human pT alpha chain, and to examine its expression at different stages of thymocyte development. Our results show that high pT alpha transcription occurs only in CD4+CD8- and CD4+CD8+ TCR alpha/beta- thymocytes, whereas it is weaker in earlier and later stages of development. Based on these results, we propose that the transition from TCR alpha/beta- CD4+CD8- to TCR alpha/beta- CD4+CD8+ thymocytes represents a critical developmental stage at which the successful expression of TCR beta promotes the clonal expansion and further maturation of human thymocytes, independent of TCR alpha.